Triboelectric (or friction) charging happens when surfaces of two different material touch and separate from each other. The atoms of one material will often have a greater attraction for electrons than the other material (difference in electron affinity). When this happens, one surface will have a negative charge (more electrons) and the other will have a positive charge (fewer electrons).
Triboelectricity is observed when friction occurs between two dielectrics, two semiconductors, or two metals. The substances may be of different chemical compositions or of identical composition but different densities. It is also observed when friction occurs, for example, between a metal and a dielectric, between two identical dielectrics, between liquid dielectrics, or between a liquid dielectric and the surface of a solid. In all cases, both substances are electrified, and their charges are equal in magnitude but opposite in signs.
Static is often a major problem on machinery that processes non-conductive materials. Two simple rules are commonly used to deal with charging problems: the first method is to ground all conductors; the second method is to either remove or control all insulative charge generators.
If the charged object is conductive it should be connected by a wire to an earth ground. This is usually the metal frame of a machine, a cold water pipe, or something similar. Alternatively, static on non-conductive surfaces can be removed by making the surface conductive to ground. Anti-static sprays and additives work this way by attracting moisture from the air and making the surface slightly conductive. There must still be a path to ground or to the oppositely charged surface. Anti-static sprays do wear-off and are sensitive to temperature and humidity levels in the air.
The ironing plate (IP) of an iron appliance usually comprises one or more metal sheets or blocks coated with certain coatings on the side which is contacting the fabric or garment during operation. The IP surface coating may serve to provide aesthetic features, protection from corrosion/mechanical damage, and/or low friction for easy gliding on fabric. Commonly used materials for the coating may include polytetrafluroethylene (PTFE), organic inorganic hybrid polymers, and porcelain enamel. Many coatings are dielectric coatings, contain insulating material or are very poor conductors of electric current. FIG. 1 is a diagram showing a structure of organic-inorganic hybrid polymer. Organic-inorganic hybrid polymers may include silicone and/or other insulative materials. R may represent an organic group such as methy, ethyl, or phenyl.
Ironing is a process whereby the ironing plate surface contacts and rubs the fabric surface continuously and repeatedly. When ironing different types of fabrics, it is unavoidable that triboelectric charging phenomenon happens, since the electron affinity of different types of fabric are not same, while the electron affinity of the ironing plate surface remains the same. If ironing is carried out with a metal ironing plate coated with the dielectric coating, the friction charging happens on the surface (i.e. charges are generated on the dielectric coating), which makes the charges hard to remove or dissipate. The dielectric coating is an insulator so that the charges has no way to release/escape. Further, as the resistivity of the dielectric coating is high, the charging speed is much faster than the releasing speed. FIGS. 2A and 2B show two scenarios of net charge on the surface of a dielectric coating coated metal ironing plate and where the metal plate is grounded by conductors from assembly side. FIG. 2A is a schematic showing a metal ironing plate 1 in which the dielectric coating 2 has a net negative charge while the ironing plate 1 is grounded. FIG. 2B is a schematic showing a metal ironing plate 1 in which the dielectric coating 2 has a net positive charge while the ironing plate 1 is grounded. The metal ironing plate 1 may be grounded by iron body 3 as shown in FIG. 2C.
The net charges on the ironing plate surface are opposite to that on the surface of treated fabric, and the net charges increases adhesion between the ironing plate 1 and the fabric. Consequently, friction forces increases. In such scenario, gliding performance is affected and user may feel that the iron is draggy during the ironing operation. In severe circumstances, the fabric may adhere to the ironing plate surface such that the iron cannot be moved relative to the fabric.
DE3617034 discloses a process for coating pressing irons with an anti-adhesive layer, and a pressing iron.